Shift levers are used in motor vehicles to achieve gear ratio changes of the transmission. For example, in a typical manual transmission automobile, a driver who is initially at a stop may shift through four gear ratios, commonly referred to as “gears” (ie., first, second, third and fourth gear) to accelerate to cruise speed. The manual transmission also has a reverse gear, in addition to the forward gears.
In that the shift lever is hand held and connected to a drive component of the vehicle, vibration from an operating engine is mechanically conducted to the shift lever, and ultimately to the hand of the driver. The idle frequency of a four cylinder engine is typically greater than 23 Hz. As the number of cylinders increases, the idle frequency will increase. Of this vibration, the lateral (that is, cross-car or side-to-side) vibration mode is the most important contribution. While the longitudinal (that is, fore-aft) vibration contribution is not very important, a subjective stiff feel of the shift lever in the driver's hand is most important in the longitudinal direction.
A shift lever can be regarded as a driven harmonic oscillator, wherein the engine vibration causes the driving. The natural vibration frequency of a shift lever in relation to the vibration frequency of an engine determines whether resonance will occur. The degree to which this can be avoided is referred to as vibration isolation. For best vibration isolation, a shift lever should have a natural vibration frequency, υS, of:υS<υE/√2,wherein υE is the vibration frequency of the engine when operating at idle. For example, if the engine has a 23 Hz vibration frequency, the shift lever should be isolated by its natural vibration frequency being less than about 16 Hz.
Prior art shift lever vibration isolators merely utilize an aluminum housing covered with rubber. The housing threadably attaches to a transmission shift and the isolator fits into a sleeve of the shift lever. Problematically, however, even though the rubber may provide damping of the amplitude of vibration, vibration isolation because of minimization of the natural vibration frequency (such as by a soft rubber interface) is contrary to the need for a subjective stiff feel (such as for example by a hard rubber interface).
Accordingly, what remains needed in the art is a shift lever vibration isolator which does not trade off minimization of the natural vibration frequency for a subjective stiff feel, and vice versa.